The invention concerns an optically variable surface pattern of the kind set forth in the classifying portion of claim 1.
Such surface patterns contain structures, generally in the form of microscopically fine relief structures, which diffract impinging light. Those diffractive patterns are suitable for example as an authenticity and security feature for enhancing the level of safeguard against forgery. They are suitable in particular for protecting value-bearing papers or security bonds, banknotes, payment means, identity cards, passes, etc.
The function thereof as an authenticity feature is to give the recipient of the article provided therewith, for example a banknote, the feeling that the article is genuine and not a forgery. The function thereof as a security feature is to prevent unauthorized copying or at least to make it extremely difficult.
Surface patterns of that kind are known from many sources: reference is made here as representative examples to EP 0 105 099 B1, EP 0 330 738 B1 and EP 0 375 833 B1. They are distinguished by the brilliance of the patterns and the movement effect in the pattern, they are embedded in a thin laminate of plastic material and they are applied, for example glued in the form of a stamp onto documents such as banknotes, bonds, personal identity papers, passports, visas, identity cards and so forth. Materials which can be used for production of the security elements are summarized in EP 0 201 323 B1.
A pixel-oriented optically variable surface pattern is known from EP 0 375 833 B1. Such a surface pattern contains a predetermined number N of different images. The surface pattern is subdivided into pixels. Each pixel is subdivided into N subpixels, wherein associated with each of the N subpixels of a pixel is an image point from one of the N images. Each subpixel contains a diffraction structure in the form of a microscopically fine relief containing information about a color value, about a stage in the brightness value and about a viewing direction. There is only ever one single image that is represented to a person viewing the surface pattern, wherein the respective visible image can be altered by tilting or rotating the surface pattern or by altering the angle of view of the viewer.
A further optically variable surface pattern is known from U.S. Pat. No. 6,157,487. In that surface pattern the microscopically fine relief structures involve a comparatively small number of lines per millimeter so that impinging light is virtually achromatically diffracted.
Another known idea is that based on the differences in spectral sensitivity of the human eye and a color photocopier, which involves providing documents with a colored background and printing information on the background in another color, wherein the information and the background involve a contrast which is perceptible to the human eye but which cannot be reproduced by the color photocopiers.